Die for molding disk substrate and method of manufacturing disk substrate

ABSTRACT

A die for molding a disk substrate capable of forming fine recessed and projected pits and grooves up to the outer periphery of the disk substrate by preventing low heat conductive elements from peeling off and burrs from occurring on the outer periphery thereof, comprising a first base die, a second base die disposed oppositely to the first base die, a first low heat conductive element fixed to the first base die, a stamper fixed onto the first low heat conductive element, a second low heat conductive element fixed onto the second base die, and a ring-like regulating member fitted to and in slidable contact with either of the first and second low heat conductive elements. The die is characterized in that the end part of the ring-shaped regulating member is positioned within the range of the outer peripheral side face of the low heat conductive element in slidable contact therewith.

TECHNICAL FIELD

The present invention relates to disk-substrate molding dies anddisk-substrate fabricating methods for forming a disk substrate.

BACKGROUND ART

Disk-shaped substrates such as optical-disk substrates and magneticoptical disks are fabricated by charging molten thermoplastic resin intoa cavity provided within a die in view of productivity. Fine convex andconcave pits and grooves are formed on a stamper made of nickel or thelike which is mounted within the die. The stamper comes into contactwith the molten thermoplastic resin so that the fine convex and concavepits and grooves on the stamper are transferred to the thermoplasticresin, and thereafter, the molten thermoplastic resin is solidified toprovide a desired optical-disk substrate (refer to JP-A No. 8-66945, forexample).

In a conventional common die construction disclosed in JP-A No. 8-66945,a fixed-side mirror-surface plate and a movable-side mirror-surfaceplate, which are controlled in the temperature, are provided at theupper and lower portions of the cavity. A stamper is mounted on themovable-side mirror-surface plate, and the stamper is secured by astamper holder at its inner side and also secured by an outer ring atits outer side.

On the other hand, as the density is increased, the intervals betweenfine convex and concave pits and grooves on the stamper are decreased,thus making it more difficult to introduce thermoplastic resin into thefine convex and concave pits and grooves. In order to transfer convexand concave pits and grooves to an optical-disk substrate, it isnecessary to raise the temperature of the stamper to above the thermaldeformation temperature of the thermoplastic resin. Since it isnecessary to further raise the maximum stamper temperature in order toincrease the flowability of the thermoplastic resin, it is necessary tofurther raise the temperature of a heat medium for adjusting the dietemperature.

However, if the temperature of the heat medium for adjusting the dietemperature is raised to a high temperature, then it will take a longtime to cool down the thermoplastic resin charged within the die to atemperature which enables extraction thereof, thus resulting in anincrease of the molding time for an optical-disk substrate. Therefore,there has been suggested a die construction employing a heat insulationsheet provided on the back surface of the stamper, in order to make thestamper temperature high even when the heat-medium temperature is low(refer to JP-A No. 62-5824, for example).

Further, there has been disclosed a die construction including lowheat-conductivity members provided within an upper die and a lower die,in order to cause the temperature within the upper and lower dies tochange in a upper-lower symmetrical manner along the thickwise direction(refer to JP-A No. 7-100866, JP-A No. 9-207141 and JP-A No. 2000-331385,for example).

As the low heat-conductivity members, heat resistant plastics such aspolyimide and ceramics have been mainly employed. JP-A No. 62-5824discloses employing aluminum and cupper as metal low heat-conductivitymaterials, and JP-A No. 2000-331385 discloses employing bismuth as a lowheat-conductivity material.

Further, conventional die constructions have malfunctions as follows.

In a die construction disclosed in JP-A No. 7-100866, a fixed-sidestamper and a movable-side stamper include a slow-cooling plate. Thefixed-side stamper and the movable-side stamper are secured by an innerholder at their inner sides and are secured by outer rings at theirouter sides. Namely, the outer surfaces of the slow-cooling plates areprotected by the outer rings. The movable-side stamper is configured andsized to be greater than the fixed-side stamper, and therefore themovable-side outer ring is positioned outside the fixed-side outer ring.Consequently, the fixed-side outer ring holds the fixed-side stamper andalso comes into contact with the outer peripheral portion of themovable-side stamper. Namely, the fixed-side outer ring is sandwichedbetween the fixed-side stamper and the movable-side stamper. Further,the fixed-side outer ring defines the outer peripheral side surface ofan optical-disk substrate. As a result, if the resin charge pressureovercomes the die fastening pressure, then the die is opened from thecontacting surface of the movable-side stamper. Consequently, resinenters into the gap between the fixed-side outer ring and themovable-side stamper, thus forming burrs on the outer peripheral sidesurface of the optical disk substrate.

In a die construction disclosed in JP-A No. 9-207141, a nest mounted ona nest-mounting portion of the die is secured by an inner pressing plateat its inner peripheral portion and is secured by an outer pressingplate at its outer peripheral portion. However, since a step is formedat the outer peripheral portion of the nest by the outer pressing plate,there is caused the problem that fine convex and concave pits andgrooves can not be formed at the outer peripheral portion of the formedoptical disk substrate.

A die construction disclosed in JP-A No. 2000-331385 is configured suchthat a movable die is slidably fit in a concave-shaped portion of afixed die. A heat insulation member is mounted on each of the movabledie and the fixed die. When the movable die slides, the outer peripheralsurface of the heat insulation member mounted on the movable die is slidwhile being fit against the inner peripheral surface of theconcave-shaped portion of the fixed die, and therefore the heatinsulation member is prone to be exfoliated from the outer peripheralside surface.

In a die construction disclosed in JP-A No. 9-262838, a heat insulationlayer made of a heat insulation polymer and a metal layer are extendedfrom the cavity surface of the base die to the side surface. However,the heat insulation layer and the metal layer provided on the sidesurface come into contact with a fixed-side mounting plate which isfaced thereto and also constitute split surfaces which can be split intotwo surfaces. Consequently, the heat insulation layer and the metallayer provided on the side surface of the base die will not serve as asliding portion during the molding operation.

Further, although plastic materials, ceramic materials and metalmaterials are conventionally employed as low-heat conductivitymaterials, these low heat-conductivity materials have problems asfollows.

Namely, plastic materials generally have poor stiffness and poor surfacestrengths. Ceramic materials are generally brittle and thus have poorimpact resistances. Aluminum and cupper (metal materials) disclosed inJP-ANO. 62-5824 have heat conductivities higher than that of thestainless steel constituting the die and therefore can not serve as lowheat-conductivity materials. Bismuth disclosed in JP-A No. 2000-331385is brittle and has poor hardness and therefore has undesirablemechanical characteristics.

DISCLOSURE OF THE INVENTION

The present invention was made in view of the aforementioned problemsand aims at providing disk-substrate molding dies and disk-substratefabricating methods which prevent exfoliation of the lowheat-conductivity members and the occurrence of burrs on the outerperiphery and enable forming fine concave and convex pits and grooves upto the outer periphery of the disk substrate.

A disk-substrate molding die according to the present inventionincludes:

a first base die;

a second base die which is placed to face with the aforementioned firstbase die;

a first low heat-conductivity member secured to the aforementioned firstbase die;

a stamper secured on the aforementioned first low heat-conductivitymember;

a second low heat-conductivity member secured on the aforementionedsecond base die; and

a ring-shaped restriction member which is fit on one of theaforementioned first low heat-conductivity member and the second lowheat-conductivity member and is in slidable contact therewith;

wherein the end portions of the aforementioned ring-shaped restrictionmember are positioned within the region of the outer peripheral sidesurface of the low heat-conductivity member which is in slidable contacttherewith. A disk-substrate fabricating method according to the presentinvention is characterized in that a disk substrate is fabricated usingthe aforementioned disk-substrate molding die.

According to the present invention, even if the resin charge pressureovercomes the die fastening pressure to slightly open the die duringcharging, it is possible to suppress the occurrence of burrs since thereis no gap at the fitting portions of the cavity which is defined by thestamper, the second low heat-conductivity member and the ring-shapedrestriction member. Further, since the outer peripheral portion of thedisk substrate is flat, fine convex and concave pits and grooves can beformed up to the possible outermost peripheral portion thereof. Further,since the unengaged end portions of the low heat-conductivity member arenot positioned at the fitting portions between the low heat-conductivitymember and the ring-shaped restriction member, it is possible to preventexfoliation of the low heat-conductivity member from the base die.

By using the aforementioned die, it is possible to suppress the increaseof the molding time even in the case of forming a disk substrate havinga higher density than conventional, since convex and concave portions onthe stamper can be transferred to the disk substrate even when thetemperature for the heat medium for adjusting the die temperature islower than the heat medium temperature for a die including noheat-conductivity member.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view of a die used in a firstembodiment of the present invention.

FIG. 2 is an enlarged cross-sectional view of an outer peripheralportion (portion X) of the die used in the first embodiment of thepresent invention.

FIG. 3 is a view illustrating the relationship between the heatconductivity of the low heat-conductivity member and the reduction ofthe temperature of the heat medium for adjusting the die temperaturewhich enables constant transfer to the optical disk, with respect to theheat-medium temperature for the case of not providing the lowheat-conductivity member.

FIG. 4 is a view illustrating the relationship between the Vickershardness and the Young's modulus of metal elements and

FIG. 5 is an enlarged cross-sectional view of an outer peripheralportion of a die used in a second embodiment of the present invention.

FIG. 6 is a schematic cross-sectional view of a die used in a thirdembodiment of the present invention.

FIG. 7 is an enlarged cross-sectional view of an outer peripheralportion of the die used in the third embodiment of the presentinvention.

FIG. 8 is an enlarged cross-sectional view of an outer peripheralportion of the die used in the fourth embodiment of the presentinvention.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, first to sixth embodiments of the present invention will bedescribed in detail with reference to attached drawings.

First Embodiment

First, a first embodiment will be described in detail with reference toFIG. 1 to FIG. 4.

FIG. 1 illustrates a cross-sectional view of main parts of adisk-substrate molding die according to the first embodiment of thepresent invention. The molding die includes a fixed die 1 and a movabledie 2. The fixed die 1 is constituted by a stamper 6, a lowheat-conductivity member 7 as a first low heat-conductivity component, afixed-side mirror-surface plate 8 as a first base die and a fixed-sidebase plate 4 which are laminated in the mentioned order from the cavity20 side.

At the center portion of the fixed die 1, there is provided a spruebushing 3 for injecting molten resin into the cavity 20. A stamperholder 5 is provided outside of the sprue bushing 3. The stamper holder5 secures the inner peripheral portions of the stamper 6 and the lowheat-conductivity member 7 with respect to the fixed-side mirror-surfaceplate 8 integrally therewith. The fixed-side base plate 4 and thefixed-side mirror-surface plate 8 are secured through bolts. The outerperipheral portions of the stamper 6 and the low heat-conductivitymember 7 are secured to the fixed-side mirror-surface plate 8 throughvacuum suction through a suction path A provided through the fixed-sidebase plate 4 and the fixed-side mirror-surface plate 8. A fixed-sidebutting ring 9 is provided at the outermost portion of the fixed-sidemirror-surface plate 8 and the fixed-side butting ring 9 is secured tothe fixed-side base plate 4.

The movable die 2 is constituted by a low heat-conductivity member 15 asa second low heat-conductivity component, a movable-side mirror-surfaceplate 16 as a second base die and a movable-side base plate 13 which arelaminated in the mentioned order from the cavity 20 side. At the centerportion of the movable die 2, there are provided an ejector pin 10, acut punch 11 and an ejector sleeve 12. The ejector pin 10 and theejector sleeve 12 are configured to protrude during removing theinjection-molded optical disk substrate from the die. The cut punch 11is for forming an inner hole in the formed optical disk. Alow-heat-conductivity-member holder 14 is provided outside of theejector sleeve 12. The inner peripheral portion of the lowheat-conductivity member 15 is secured to the movable-sidemirror-surface plate 16 by the low-heat-conductivity-member holder 14.The outer peripheral portion and the side surface portion of the lowheat-conductivity member 15 are secured with respect to the movable-sidemirror-surface plate 16 through vacuum suction through a suction path Bprovided through the movable-side base plate 13 and the movable-sidemirror-surface plate 16.

FIG. 2 is an enlarged view of the portion X in FIG. 1.

The low heat-conductivity member 15 covers the upper surface and theouter side surface of the movable-side mirror-surface plate 16. Anunengaged end portion 21 of the outer side surface 29 of the lowheat-conductivity member 15 extends up to a portion just before themovable-side base plate 13.

In the fixed-side base plate 4 and the movable-side base plate 13, thereare provided paths for flowing a heat medium such as water therethroughfor adjusting the die temperature of the fixed die 1 and the movable die2. By heating or cooling the heat medium from outside, the temperaturesof the fixed die 1 and the movable die 2 are adjusted to predeterminedtemperatures.

Outside of the low heat-conductivity member 15, there is configured anouter ring 17 for defining the outer peripheral side surface of theoptical-disk substrate such that it is fit on the low heat-conductivitymember 15. The outer ring 17 is biased toward the fixed die 1 by acompressing spring 22. In the surface of the movable-side base plate 13,there is formed an annular slot 23 shaped to extend along the unengagedend portion 21 of the low heat-conductivity member 15 which is thelowermost end portion of the outer peripheral side surface 29 thereof.This annular slot 23 prevents the unengaged end portion 21 from comingin contact with the movable-side base plate 13 due to thermal expansionof the low heat-conductivity member 15, even though the unengaged endportion 21 of the low heat-conductivity member 15 extends to thevicinity of the movable-side base plate 13. This can prevent exfoliationof the low heat-conductivity member 15 from the movable-sidemirror-surface plate 16.

A movable-side butting ring 18 is provided outside of the outer ring 17.The compression spring 22 of the outer ring 17 and the movable-sidebutting ring 18 are both secured to the movable-side base plate 13.Consequently, when the movable die 2 is moved vertically, the outer ring17 and the movable-side butting ring 18 are moved integrally with themovable-side base plate 13. When the fixed die 1 and the movable die 2are closed, the movable-side butting ring 18 is butted against thefixed-side butting ring 9, thus defining the height of the cavity 20,namely the thickness of the optical-disk substrate.

The inner peripheral side surface 27 of the annular-shaped outer ring 17is in slidable contact with the outer peripheral side surface 29 of thelow heat-conductivity member 15. The inner peripheral side surface 27 ofthe outer ring 17 and the outer peripheral side surface 29 of the lowheat-conductivity member 15 are in slidable contact with each other at aslidable contact surface 24. They are configured to engage with eachother such that the lower end portion 26 of the outer ring 17 is belowthe molding surface 28 of the lower heat-conductivity member 15 which isthe uppermost surface thereof and the lower end portion 26 of the outerring 17 is positioned above the unengaged end portion 21 of the lowheat-conductivity member 15 which is the lowermost end portion thereof.Namely, the lower end portion of the outer ring 17 is positioned withinthe region of the outer peripheral side surface 29 of the lowheat-conductivity member 15 which is in slidable contact therewith. Eventhough the outer ring 17 is slightly slid in the vertical directionalong the outer peripheral side surface 29 of the low heat-conductivitymember 15, it will not be disengaged from the outer peripheral sidesurface 29 of the low heat-conductivity member 15.

Next, investigations were conducted for the heat conductivity which thelow heat-conductivity components, namely the low heat-conductivity plate7 and the low heat-conductivity member 15, are required to have. Thefixed-side mirror-surface plate 8 and the movable-side mirror-surfaceplate 16 are made of a stainless steel. The fixed-side mirror-surfaceplate 8 and the movable-side mirror-surface plate 16 have a heatconductivity of about 25 W/m·K. On the other hand, experiments wereconducted for the case of fabricating the low heat-conductivity plate 7and the low heat-conductivity member 15 from a stainless steel and forthe case of increasing the thicknesses of the fixed-side mirror-surfaceplate 8 and the movable-side mirror-surface plate 16 instead ofproviding the low heat-conductivity plate 7 and the lowheat-conductivity member 15. Namely, injection moldings were performedusing a stamper with a pit density of 40 Gbit/in² under a conditionwhere the heat-medium temperatures for adjusting the temperatures of thefixed die 1 and the movable die 2 were made equal to determine the heatmedium temperature which could realize sufficient transferability. Thestamper was made of nickel and had a heat conductivity of 90 W/m·K. Thethermoplastic resin was a polycarbonate resin and the molding cycle was10 seconds. Further, the temperatures at the stamper surface and themirror surface were measured before and after the start of molding todetermine the heat conductivities among the members. On the basis of thedata, the heat-medium temperature which could raise the stampertemperature, at a maximum, to the same temperature as that of the caseof not providing the low heat-conductivity plate 7 and the lowheat-conductivity member 15 was determined. FIG. 3 illustrates theresult.

FIG. 3 shows that the heat-medium temperature can be reduced by 10 K ormore than the case of not providing the low heat-conductivity plate 7and the low heat-conductivity member 15 as low heat-conductivitycomponents when the low heat-conductivity plate 7 and the lowheat-conductivity member 15 have a heat conductivity of 15 W/m·K orless. Accordingly, it is necessary that the low heat-conductivity member15 has a heat conductivity of 15 W/m·K or less.

Next, investigations were conducted for the mechanical characteristicsthat the low heat-conductivity member 15 is required to have. Pure-metalplates with a thickness of 2 mm were prepared and experiments fordetermining the presence or absence of deformations and surface flawswere conducted. The plates were subjected, ten thousand times, to a loadcorresponding to the resin charge pressure which was applied duringmolding and thereafter it was determined whether or not deformation wascaused therein. Further, scratching tests using a cloth were conductedand then it was determined whether or not flaws were caused thereon.

Table. 1 illustrates the results of flaws and deformation. There is alsoillustrated in Table. 1 Young's modulus data since there is acorrelation between deformation and Young's modulus indicating therigidity of material. Further, there is also illustrated Vickershardness data since there is a correlation between flaws and the Vickershardness which indicates the surface hardness of material. Further,there is also illustrated the data of the stainless steel constitutingthe die main body. TABLE 1 Young's Modulus Vickers Material (GPa)Hardness Deformation Flaws Aluminum 67 30 Present Present Gold 78 40Present Present Cupper 115 60 Absent Absent Platinum 168 80 AbsentAbsent Titanium 106 110 Absent Absent Nickel 203 120 Absent Absent Iron190 125 Absent Absent Stainless 199 200 Absent Absent Steel

From these results, it can be seen that the low heat-conductivity membermade of a pure metal material preferably has a Young's modulus of 100GPa or more and a Vickers hardness of 50 or more.

There are relationships between the Vickers hardnesses and the Young'smoduluses of pure metal materials other than stainless steel, as in FIG.4. From FIG. 4, it can be seen that the Vickers hardness tends to beincreased with increasing Young's modulus and, when the Young's modulusis equal to or greater than 100 GPa, the Vickers hardness is equal to orgreater than 50. Consequently, it is preferable that the lowheat-conductivity member 15 has a Young's modulus of 100 GPa or more.

However, there exists no pure metal material having a heat conductivityof 15 W/m·K or less and a Young's modulus of 100 GPa or more.

On the other hand, an alloy has a Young's modulus close to that of thepure metal which is the main ingredient thereof and has a surfacehardness higher than that of the pure metal. As a result of earneststudies of various metal materials, it has been found that titaniumalloys mainly consisting of titanium have heat conductivities within therange of 7 to 8 W/m·K.

Further, among titanium alloys, alpha-beta alloys and beta alloys haveYoung's moduluses within the range of 100 to 130 GPa and have Vickershardnesses of 240 or more as the surface hardnesses. Consequently, itcan be seen that titanium alloys exhibit Vickers hardnesses equivalentto that of the stainless steel constituting the die main body and thusare sufficiently hard.

The fixed die 1 and the movable die 2 are positioned with the fitbetween the fixed-side butting ring 9 and the movable-side butting ring18. Namely, the positioning in the direction towards the center isperformed with the taper portions provided on the fixed-side buttingring 9 and the movable-side butting ring 18 while the positioning in thethickwise direction is performed with the flat portions thereof. Thisresults in a configuration capable of suppressing wear due to slide ofthe outer ring 17 with respect to the low heat-conductivity member 15.The greater the hardness of the materials of them, the more effectivelywear can be suppressed. It has been proven that, when the outer ring 17is made of a stainless steel similarly to the other die members, if thelow heat-conductivity member 15 has a Vickers hardness of 100 or more,then there will be no practical problem. Consequently, it is preferablethat the low heat-conductivity member 15 has a Vickers hardness of 100or more. Obviously, it is possible to form the outer ring 17 from thesame material as that of the low heat-conductivity plate 7 and the lowheat-conductivity member 15.

The low heat-conductivity plate 7 and the heat-conductivity member 15 asthe low heat-conductivity components are made of Ti-6Al-4V, which is AMS(Aerospace Material Specification) 4911. This material is an alloymaterial containing 5.50 to 6.75 wt. % aluminum and 3.50 to 4.50 wt. %vanadium and the balance thereof consists of titanium. This material hasa Young's modulus of 113 GPa. The thickness thereof is 2 mm.

Injection moldings were performed using a stamper with a pit density of40 Gbit/in² under a condition where the die temperatures of the fixeddie 1 and the movable die 2, namely the heat-medium temperaturesthereof, were made equal. Then, the heat medium temperature whichenabled sufficiently transferring a predetermined pit shape wasdetermined. The used thermoplastic resin was a polycarbonate resin andthe molding cycle was 10 seconds. Experiments of injection-molding wereconducted by varying the heat-medium temperature in steps of 5 K. As aresult, it was proven that the predetermined pit shape could be obtainedeven when the heat medium temperature was lower by about 15 K than thecase of not providing the low heat-conductivity plate 7 and the lowheat-conductivity member 15.

It is preferable that the low heat-conductivity plate 7 and the lowheat-conductivity member 15 have thicknesses of 2 mm or more in view ofensuring workability. When the low heat-conductivity plate 7 and the lowheat-conductivity member 15 have thicknesses of 2 mm or more, the resininjected into the cavity 20 of the die disperses heat at substantially aconstant initial rate. As the thicknesses of the low heat-conductivityplate 7 and the low heat-conductivity member 15 are increased, the heatcapacities of the low heat-conductivity plate 7 and the lowheat-conductivity member 15 are increased, which hinders cooling of thelow heat-conductivity plate 7 and the low heat-conductivity member 15resulting in an increase of the fabrication time of an optical-disksubstrate. Accordingly, the low heat-conductivity plate 7 and the lowheat-conductivity member 15 have thicknesses of 10 mm or less andpreferably have thicknesses of 5 mm or less.

The aforementioned titanium alloy may contain aluminum, vanadium,molybdenum, iron and chromium, in addition to titanium. Aluminum has theproperty of enhancing the creep strength. Vanadium, molybdenum, iron andchromium have the property of increasing the Young's modulus and theproperty of improving the workability. It is preferable that thealuminum content is within the range of 2 wt. % to 9 wt. % and thevanadium content is within the range of 2 wt. % to 16 wt. % in view ofthe mechanical characteristics and the workability. It is morepreferable that the aluminum content is within the range of 2 wt. % to 7wt. % and the vanadium content is within the range of 2 wt. % to 6 wt.%. In this case, it is possible to form a thin plate member havingsufficient mechanical characteristics and workability. If the aluminumcontent is greater than the aforementioned content, the Young's moduluswill be degraded and, if the vanadium content is greater than theaforementioned content, the workability will be degraded. Further, ifany of the aluminum content and the vanadium content is smaller than theaforementioned content, it is impossible to offer the effects thereof.

It is preferable that the low heat-conductivity plate members 7 and 15have surface roughness (center roughness) of 0.2 micrometer or less, inview of ensuring the flatness of the surface of the optical-disksubstrate to which transfer is performed with the stamper 6.

If the surface roughness of the low heat-conductivity plates 7 and 15 isgreater than 0.2 micrometer as the center roughness, undulations may betransferred to the surface of the formed optical-disk substrate, whichmay result in degradation of focus signals and tracking signals of theoptical-disk substrate thus preventing sufficient reading of signals.Obviously, while the surface roughness of the low heat-conductivitymember 15 has influences only during recording, erasing or reproducingof data onto or from the disk substrate through laser light passedthrough the optical-disk substrate, the surface roughness of the lowheat-conductivity member 15 hardly has influences in the case ofrecording, erasing or reproducing of data through laser light which isincident from the opposite side without being passed though theoptical-disk substrate.

Since there are provided the low heat-conductivity plate 7 and the lowheat-conductivity member 15 as low heat-conductivity components at theupper and lower portion of the cavity 20 of the die, respectively, it isnot necessary to significantly increase the heat medium temperature foradjusting the die temperature. This enables fabrication of ahigh-density optical-disk substrate within a molding time equivalent tomolding times which have been required for those having conventionaldensities, without increasing surface deflections and warpage.

Further, the inner peripheral side surface 27 of the outer ring 17 whichdefines the outer peripheral side surface of the optical-disk substrateis fitted on the outer peripheral side surface 29 of the lowheat-conductivity member 15 which forms the lower surface of the cavity20. Consequently, even if the resin charge pressure overcomes the diefastening pressure and thus the die is slightly opened when molten resinhas been charged, no gap will be generated in the cavity 20 since theouter ring 17 is biased towards the fixed die 1 by the compressionspring 22. This prevents the occurrence of burrs on the outer peripheralside surface of the optical disk substrate.

Further, the low heat-coductivity member 15 which is in contact with theslidable outer ring 17 is constituted by a member having sufficientmechanical characteristics, which eliminates the possibility of failuresof the low heat-conductivity member 15 due to wear of the slidablecontact surface 24 of the low heat-conductivity member 15.

Further, the unengaged end portion 21 of the low heat-conductivitymember 15 which is the lowermost end portion of the outer peripheralside surface 29 thereof is positioned below the lower end portion 26 ofthe outer ring 17 and the lower end portion 26 of the inner peripheralside surface 27 of the outer ring 17 is always overlapped with the outerperipheral side surface 29 of the low heat-conductivity member 15.Namely, the lower end portion 26 of the outer ring 17 is within theregion of the outer peripheral side surface 29 of the lowheat-conductivity member 15 which is in slidable contact therewith,which prevents the side surface of the low heat-conductivity member 15from riding up and exfoliating from the movable-side mirror-surfaceplate 16.

Further, the free flat portion extends up to the outer peripheralportion of the cavity 20, and therefore fine concave and convex pits andgrooves will be formed up to the possible outermost peripheral portionof the optical-disk substrate formed by injection molding.

Further, since the low heat-conductivity member 7 as a lowheat-conductivity component is not directly faced to the cavity 20 andalso it will not be slid and rubbed, it is possible to form the lowheat-conductivity member 7 from the same material as that of the lowheat-conductivity member 15 as a low heat-conductivity component,although it is not necessary.

Further, while the low heat-conductivity member 7 and the stamper 6 ofthe fixed-side die 1 are vacuum-suctioned through the common suctionpath A, it is also possible to vacuum-suction them individually throughseparate suction paths.

Further, while a polycarbonate resin is used as the thermoplastic resin,it is possible to employ a polyolefin resin, an acrylic resin and otherresins.

Second Embodiment

FIG. 5 illustrates a detailed cross-sectional view of an outer portionof a disk-substrate molding die according to a second embodiment of thepresent invention. The disk-substrate molding die is similar to thataccording to the first embodiment at the point that the lowheat-conductivity member 15 extends to the outer peripheral side surfaceof the movable-side mirror-surface plate 16 and the lower end portion 26of the outer ring 17 is positioned above the unengaged end portion 21 ofthe low heat-conductivity member 15 which is the lowermost surfacethereof. However, in FIG. 5, the disk-substrate molding die isconfigured such that, even if the outer ring 17 slides, the lower endportion 26 of the outer ring 17 will be stopped at a portion above theunengaged end portion 21 of the low heat-conductivity member 15 and willnot reach the position of the bonding surface at which the lowheat-conductivity member 15 is bonded to the movable-side mirror-surfaceplate 16. Consequently, similarly, the lower end portion 26 of the outerring 17 lies within the region of the outer peripheral side surface 29of the low heat-conductivity member 15 which is in slidable contacttherewith, which eliminates the possibility of exfoliation of the lowheat-conductivity member 15 from the movable-side mirror-surface plate16.

Further, by employing a material having a Young's modulus of 100 GPa ormore for the low heat-conductivity member 15, it is possible toeliminate the possibility of failures due to wear of the portion whichslides against the outer ring 17. Further, similarly to in the firstembodiment, the outer peripheral portion of the cavity 20 is made flatand therefore fine concave and convex pits and grooves will be formed upto the possible outermost peripheral portion of the optical-disksubstrate formed by injection molding.

Further, since there are provided the low heat-conductivity plate 7 andthe low heat-conductivity member 15 at the upper and lower portions ofthe cavity 20 within the die, it is not necessary to significantly raisethe temperature of the heat medium for adjusting the die temperature.This enables fabrication of a high-density optical-disk substrate withina molding time equivalent to molding times which have been required forthose having conventional densities, without increasing surfacedeflections and warpage. Further, the outer ring 17 which defines theouter peripheral side surface of the optical-disk substrate is fitaround the low heat-conductivity member 15 forming a single wall surfaceof the cavity 20. Consequently, even if the resin charge pressureovercomes the die fastening pressure and thus the die is slightly openedwhen molten resin has been charged, no gap will be generated in thecavity 20 since the outer ring 17 is biased towards the fixed die 1 bythe compression spring 22. This prevents the occurrence of burrs on theouter peripheral side surface of the optical disk substrate.

Third Embodiment

FIG. 6 is a schematic cross-sectional view of a disk-substrate moldingdie according to a third embodiment of the present invention. FIG. 7illustrates an enlarged detailed view of the portion X at the outerperipheral portion of the die.

The disk-substrate molding die according to the third embodiment isdifferent from that of the first embodiment in the method of securingthe outer ring 17 and the method of securing the low heat-conductivityplate 7 and the low heat-conductivity member 15.

A non-through thread hole 42 is provided in the upper surface of the lowheat-conductivity plate 7 at an outer portion thereof. A through hole 41is provided in the fixed-side mirror-surface plate 8 at the positionwhich coincides with the aforementioned thread hole 42. The lowheat-conductivity plate 7 is secured to the fixed-side mirror-surfaceplate 8 through a thread 32 screwed into the thread hole 42.

A non-through thread hole 44 is provided in the lower surface of thefixed-side mirror-surface plate 8 at an outer portion thereof. A throughhole 43 is provided through the outer ring 17 at the position whichcoincides with the aforementioned thread hole 44. The outer ring 17 issecured to the fixed-side mirror-surface plate 8 through a thread 34screwed into the thread hole 44.

A non-through thread hole 46 is provided in the upper surface of the lowheat-conductivity member 15 in the movable die 2 at an outer portionthereof. A through hole 45 is provided in the fixed-side mirror-surfaceplate 16 at the position which coincides with the aforementioned threadhole 46. The low heat-conductivity member 15 is secured to themovable-side mirror-surface plate 16 through a thread 36 screwed intothe thread hole 46.

A countersink hole 47 is provided in the outer peripheral wall surface29 of the low heat-conductivity member 15 in the movable die 2 at alower portion thereof. A non-through hole 48 is provided in themovable-side mirror-surface plate 16 at the position which coincideswith the aforementioned countersink hole 47. The outer peripheral sidesurface 29 of the low heat-conductivity member 15 is secured to themovable-side mirror-surface plate 16 through a thread 38 screwed intothe thread hole 48 in an embedded manner. Consequently, even if theinner peripheral side surface 27 of the outer ring 17 is slid againstthe outer peripheral side surface 29 of the low heat-conductivity member15, it will not rub against the head of the thread 38.

By using the disk-substrate molding die according to the thirdembodiment, it is also possible to form an optical-disk substrate byinjection molding of a thermoplastic resin such as polycarbonate andpolyolefin.

According to the third embodiment, similarly, since there are providedthe low heat-conductivity plate 7 and the low heat-conductivity member15 as low heat-conductivity components at the upper and lower portion ofthe cavity 20 of the die, respectively, it is not necessary tosignificantly increase the temperature of the heat medium for adjustingthe die temperature. This enables fabrication of a high-densityoptical-disk substrate within a molding time equivalent to molding timeswhich have been required for those having conventional densities,without increasing surface deflections and warpage. Further, the outerring 17 which defines the outer peripheral side surface of theoptical-disk substrate is fit on the low heat-conductivity member 15forming the lower surface of the cavity 20. Consequently, even if theresin charge pressure overcomes the die fastening pressure and thus thedie is slightly opened when molten resin has been charged, no gap willbe generated in the cavity 20 since the outer ring 17 is secured on thefixed die 1. This prevents the occurrence of burrs on the outerperipheral side surface of the optical disk substrate. Further, the lowheat-conductivity member 15 which is in contact with the slidable outerring 17 is constituted by a member having sufficient mechanicalcharacteristics, which eliminates the possibility of failures of the lowheat-conductivity member 15 due to wear of the slidable contact surface24 of the low heat-conductivity member 15. Further, the unengaged endportion 21 of the low heat-conductivity member 15 which is the lowermostend portion of the outer peripheral side surface 29 thereof ispositioned below the lower end portion 26 of the outer ring 17 and thelower end portion 26 of the outer ring 17 is always overlapped with thelow heat-conductivity member 15, thereby preventing the side surface ofthe low heat-conductivity member 15 from riding up and exfoliating fromthe movable-side mirror-surface plate 16. Further, the free flat portionextends up to the outer peripheral portion of the cavity 20, andtherefore fine concave and convex pits and grooves will be formed up tothe possible outermost peripheral portion of the optical-disk substrateformed by injection molding.

Further, since the low heat-conductivity member 7 is not directly facedto the cavity 20 and also it will not be slid and rubbed, it is possibleto form the low heat-conductivity member 7 from the same material asthat of the low heat-conductivity member 15 as a low heat-conductivitycomponent, although it is not necessary.

According to the third embodiment, since the outer peripheral portionsof the stamper 6 and the low heat-conductivity member 7 are secured tothe fixed-side mirror-surface plate 8 through the thread 34 for securingthe outer ring 17, it is not necessary to secure the stamper 6 throughvacuum suction through the suction path A. While a spot facing hole anda countersink hole are provided as thread-mounting portions, it is alsopossible to interchange these holes or provide completely the sameholes.

Forth Embodiment

FIG. 8 illustrates a detailed cross-sectional view of an outerperipheral portion of a disk-substrate molding die according to a forthembodiment of the present invention. The disk-substrate molding dieaccording to the forth embodiment is configured such that the outerperipheral side surface 29 of the low heat-conductivity member 15secured on the movable-side mirror-surface plate 16 is in slidablecontact with the inner peripheral side surface 27 of the outer ring 17secured to the fixed-side mirror-surface plate 8.

The disk-substrate molding die according to the forth embodiment issimilar to the disk-substrate molding die according to the thirdembodiment in that the low heat-conductivity member 15 extends to theouter peripheral portion of the movable-side mirror-surface plate 16 andthe lower end portion 26 of the outer ring 17 is positioned above theunengaged end portion 21 of the low heat-conductivity member 15 which isthe lowermost surface thereof. However, in FIG. 5, the disk-substratemolding die is configured such that the lower end portion 26 of theouter ring 17 will be stopped at a portion above the unengaged endportion 21 of the low heat-conductivity member 15 and will not reach thebonding surface between the low heat-conductivity member 15 and themovable-side mirror-surface plate 16, even if the movable die 2 isvertically moved. Consequently, in this case, similarly, the lower endportion 26 of the outer ring 17 lies within the region of the outerperipheral side surface 29 of the low heat-conductivity member 15 whichis in slidable contact therewith, which eliminates the possibility ofexfoliation of the low heat-conductivity member 15 from the movable-sidemirror-surface plate 16.

Further, by employing a material having a Young's modulus of 100 GPa ormore as the material of the low heat-conductivity member 15, it ispossible to eliminate the possibility of failures due to wear of theportion of the low heat-conductivity member 15 which slides against theouter ring 17. Further, similarly to in the first embodiment, the outerperipheral portion of the cavity 20 is made flat and therefore fineconcave and convex pits and grooves can be formed up to the possibleoutermost peripheral portion of the optical disk substrate formed byinjection molding.

Further, since the low heat-conductivity plate 7 and the lowheat-conductivity member 15 are provided at the upper and lower portionsof the cavity 20 within the die, it is not necessary to significantlyraise the temperature of the heat medium for adjusting the dietemperature. This enables fabrication of a high-density optical-disksubstrate within a molding time equivalent to molding times which havebeen required for those having conventional densities, withoutincreasing surface deflections and warpage. Further, the die isconfigured such that the outer ring 17 which defines the outerperipheral side surface of the optical disk is fit on the lowheat-conductivity member 15 forming one of the wall surfaces of thecavity 20. Consequently, even if the resin charge pressure overcomes thedie fastening pressure to slightly open the die after charging themolten resin, no gap will be generated within the cavity 20 since theouter ring 17 is biased towards the fixed die 1. This prevents theoccurrence of burrs on the outer peripheral side surface of the opticaldisk substrate.

Fifth Embodiment

A disk-substrate molding die according to a fifth embodiment of thepresent invention is characterized in that the surface of the lowheat-conductivity member 15 is covered with a metal material with highstiffness in comparison with the disk-substrate molding dies accordingto the first to forth embodiments. Namely, it is covered with a metalmaterial with high stiffness, in order to facilitate the maintenance ofthe mirror surface of the titanium alloy used as the lowheat-conductivity member 15. It is preferable that this coating layerhas more excellent mechanical characteristics than those of the titaniumalloy. The coating layer preferably has a Young's modulus of 150 GPa orhigher and more preferably has a Young's modulus of 200 GPa or higher.Metal materials such as nickel, chromium, tungsten, molybdenum and thelike are suitable. The coating layer is constituted by at least a singlematerial selected from a group consisting of nickel, chromium, tungsten,and molybdenum. As the coating method, vacuum deposition, sputtering,plating and the like may be employed. A coating layer formed by surfacetreatment such as plating tends to have greater hardness than bulkproducts. For example, an electroplated nickel has a Vickers hardness ofabout 300 and an electroless-plated nickel has a Vickers hardness ofabout 500 while nickel bulks have a Vickers hardness of 120 according toTable. 1. Further, by applying heat treatment to an electroless-platedcoating layer at a temperature within the range of 350 to 400 degree. C,the Vickers hardness thereof is increased to about 900. An electroplatedchromium has a Vickers hardness of about 1000, while chromium bulksgenerally have a Vickers hardness of about 400. In order to maintain themirror-surface quality of the low heat-conductivity member 15, a hardcoating layer is formed on the low heat-conductivity member 15. As thecoating method for forming the coating layer, electroplating orelectroless-plating methods are preferable. Also, it is preferable thatheat treatment is applied to the coating layer after the coating layeris formed by a plating process or the like.

After the formation of the hard coating layer on the surface of the lowheat-conductivity member 15, polishing is applied thereto using a buffor the like to finish the surface of the coating layer into a mirrorsurface. It is preferable that the coating layer has a surface roughnessof 0.2 micrometer or less as a center roughness. This is for suppressingundulations and the like on the surface of the injection-molded opticaldisk substrate to alleviate the influence thereof on the transmissionand the reflection of light, thus improving the signal quality.

The disk-substrate molding die according to the fifth embodiment can beused to perform injection molding of a thermoplastic resin, such aspolycarbonate and polyolefin, to form an optical disk substrate.

According to the fifth embodiment, similarly, since the lowheat-conductivity plate 7 and the low heat-conductivity member 15 areprovided at the upper and lower portion of the cavity 20 of the dies,respectively, it is not necessary to significantly raise the temperatureof the heat medium for adjusting the die temperature. This enablesfabrication of a high-density optical-disk substrate within a moldingtime equivalent to molding times which have been required for thosehaving conventional densities, without increasing surface deflectionsand warpage. Further, the die is configured such that the outer ring 17which defines the outer peripheral side surface of the optical disksubstrate is fitted on the low heat-conductivity member 15 forming thelower surface of the cavity 20. Consequently, even if the resin chargepressure overcomes the die fastening pressure to slightly open the dieafter charging the molten resin, no gap will be generated within thecavity 20 since the outer ring 17 is secured to the fixed die 1. Thisprevents the occurrence of burrs on the outer peripheral side surface ofthe optical disk substrate. Further, the low heat-conductivity member 15which is in contact with the slidable outer ring 17 is constituted by amember having sufficient mechanical characteristics, which eliminatesthe possibility of failures of the low heat-conductivity member 15 dueto were of the sliding surface 24 of the low heat-conductivity member15. Further, the unengaged end portion 21 of the low heat-conductivitymember 15 which is the lowermost end portion of the outer peripheralside surface 29 thereof is positioned below the lower end portion 26 ofthe outer ring 17 and the lower end portion 26 of the outer ring 17 isalways overlapped with the low heat-conductivity member 15, therebypreventing the side surface of the low heat-conductivity member 15 fromriding up and exfoliating from the movable-side mirror-surface plate 16.Further, the free flat portion extends to the outer peripheral portionof the cavity 20, and therefore fine concave and convex pits and groovescan be formed up to the possible outermost peripheral portion of theinjection-molded optical disk substrate.

While a metal coating layer is provided only on the lowheat-conductivity member 15 in this case, it is obvious that such ametal coating layer may be provided on the low heat-conductivity plate7. In this case, it is necessary that they have substantially the samethermal expansion coefficient for preventing exfoliation, and thereforeit is preferable that the low heat-conductivity plate 7 is made ofmetal.

Sixth Embodiment

In a disk-substrate molding die according to a sixth embodiment of thepresent invention, a lubricating thin layer made of DLC (damond-likecarbon) or a lubricating thin layer made of a material containingfluorine is provided on the metal coating layer on the lowheat-conductivity member 15 described in the fifth embodiment. This canreduce the friction coefficient of the fitting surface between the lowheat-conductivity member 15 and the outer ring 17 during sliding, thusoffering the effect of improving the durability of both the lowheat-conductivity member 15 and the outer ring 17. Also, it is possibleto provide a thin layer made of DLC (diamond-like carbon) or a thin filmmade of a material containing fluorine, directly on the surface of thelow heat-conductivity plate 7 or the low heat-conductivity member 15. Asdescribed above, the aforementioned lubricating thin layer may beprovided either on the surface of the low heat-conductivity plate 7 orthe low heat-conductivity member 15 which is a low heat-conductivitycomponent or on the surface of the metal coating layer.

Obviously, according to the sixth embodiment, since the lowheat-conductivity plate 7 and the low heat-conductivity member 15 areprovided at the upper and lower portion of the cavity 20 of the dies,respectively, it is not necessary to significantly raise the temperatureof the heat medium for adjusting the die temperature. This enablesfabrication of a high-density optical-disk substrate within a moldingtime equivalent to molding times which have been required for thosehaving conventional densities, without increasing surface deflectionsand warpage. Further, the outer ring 17 which defines the outerperipheral side surface of the optical disk substrate is fitted on thelow heat-conductivity member 15 forming the lower surface of the cavity20. Consequently, even if the resin charge pressure overcomes the diefastening pressure to slightly open the die after charging the moltenresin, no gap will be generated within the cavity 20 since the outerring 17 is secured to the fixed die 1. This prevents the occurrence ofburrs on the outer peripheral side surface of the optical disksubstrate. Further, the low heat-conductivity member 15 which is incontact with the slidable outer ring 17 is constituted by a memberhaving sufficient mechanical characteristics, which eliminates thepossibility of failures of the low heat-conductivity member 15 due towear of the sliding surface 24 of the low heat-conductivity member 15.Further, the unengaged end portion 21 of the low heat-conductivitymember 15 which is the lowermost end portion of the outer sideperipheral surface 29 thereof is positioned below the lower end portion26 of the outer ring 17 and the lower end portion 26 of the outer ring17 is always overlapped with the low heat-conductivity member 15,thereby preventing the side surface of the low heat-conductivity member15 from riding up and exfoliating from the movable-side mirror-surfaceplate 16. Further, the free flat portion extends to the outer peripheralportion of the cavity 20, and therefore fine concave and convex pits andgrooves can be formed up to the possible outermost peripheral portion ofthe injection-molded optical disk substrate.

In the first and second embodiments, the low heat-conductivity plate 7and the low heat-conductivity member 15 are secured to the base dieswhich are the fixed-side mirror-surface plate 4 and the movable-sidemirror-surface plate 16 through vacuum suction. In the third and forthembodiments, the low heat-conductivity plate 7 and the lowheat-conductivity member 15 are secured to the base dies which are thefixed-side mirror-surface plate 4 and the movable-side mirror-surfaceplate 16 by fastening with threads. Also, the low heat-conductivityplate 7 and the low heat-conductivity member 15 may be secured to thebase dies which are the fixed-side mirror-surface plate 4 and themovable-side mirror-surface plate 16 through both vacuum suction andfastening with threads.

While in the present embodiments there have been described cases ofinstalling the stamper in the fixed-side die, it is obvious that thestamper may be installed in the movable-side die.

While in the present embodiments there have been described cases wherethe injection-molded product is an optical-disk substrate, the presentinvention is applicable to any products having a disk shape.

While in the aforementioned embodiments there have been described aninjection-molded product formed by melting a thermoplastic resin outsideof the die and then injecting the molten resin into the die, the presentinvention is applicable to compression-molded products fabricated bymelting a thermoplastic rein within the heated die and then solidifyingit. Then, fabrication methods employing the dies according to thepresent invention are included in the scope of the claims of the presentapplication.

INDUSTRIAL APPLICABILITY

The disk-substrate molding dies and the disk-substrate fabricatingmethods according to the present invention are usable for moldingmethods for molding thermoplastic resin using a die to form an opticaldisk substrate or the like. Further, they are applicable to applicationssuch as processes for glossing the surface of thermoplastic resin.

1. A disk-substrate molding die including: a first base die; a secondbase die which is placed to face with the first base die; a first lowheat-conductivity member secured to the first base die; a stampersecured on the first low heat-conductivity member; a second lowheat-conductivity member secured on the second base die; and aring-shaped restriction member which is fit on one of the first lowheat-conductivity member and the second low heat-conductivity member andis in slidable contact therewith; wherein the end portions of thering-shaped restriction member are positioned within the region of theouter peripheral side surface of the low heat-conductivity member whichis in slidable contact therewith.
 2. The disk-substrate molding die ofclaim 1, wherein the low heat-conductivity member which is in slidablecontact with the ring-shaped restriction member is a plate-like body. 3.The disk-substrate molding die of claim 1, wherein the lowheat-conductivity member which is in slidable contact with thering-shaped restriction member covers the outer peripheral side surfaceof the base die.
 4. The disk-substrate molding die of claim 3, whereinthe base die has a slot which receives the extending end portion of thelow heat-conductivity member when the heat-conductivity member extendsto the position which covers wholly the outer peripheral side surface ofthe base die.
 5. The disk-substrate molding die of claim 1, wherein thelow heat-conductivity member has a Young's modulus of 100 GPa or more.6. The disk-substrate molding die of claim 5, wherein the lowheat-conductivity member has a heat conductivity of 15 W/m·K or less. 7.The disk-substrate molding die of claim 5, wherein the lowheat-conductivity member has a Vickers hardness of 100 or more.
 8. Thedisk-substrate molding die of claim 5, wherein the low heat-conductivitymember mainly consists of titanium.
 9. The disk-substrate molding die ofclaim 8, wherein the low heat-conductivity member has an aluminumcontent within the range of 2 wt. % to 9 wt. % and a vanadium contentwithin the range of 2 wt. % to 16 wt. %.
 10. The disk-substrate moldingdie of claim 1, wherein the low heat-conductivity member has a thicknesswithin the range of 2 mm to 10 mm.
 11. The disk-substrate molding die ofclaim 1, wherein the low heat-conductivity member is secured to the basedie through vacuum suction.
 12. The disk-substrate molding die of claim1, wherein the low heat-conductivity member is secured to the base diethrough a thread.
 13. The disk-substrate molding die of claim 1, whereinthe first and second low heat-conductivity members are formed from thesame material.
 14. The disk-substrate molding die of claim 1, whereinthe low heat-conductivity member has a surface roughness of 0.2micrometer or less as a center roughness.
 15. The disk-substrate moldingdie of claim 1, wherein at least the surface of the second lowheat-conductivity member is covered with a coating layer which consistsof metal material having a Young's modulus of 150 GPa or higher andmore.
 16. The disk-substrate molding die of claim 15, wherein thecoating layer is constituted by at least a single material selected froma group consisting of nickel, chromium, tungsten, and molybdenum. 17.The disk-substrate molding die of claim 15, wherein the coating layer isformed by a plating process.
 18. The disk-substrate molding die of claim15, wherein the coating layer is treated with heat.
 19. Thedisk-substrate molding die of claim 15, wherein the coating layer has asurface roughness of 0.2 micrometer or less as a center roughness. 20.The disk-substrate molding die of claim 1, wherein the surface of thesecond low heat-conductivity member is covered with a thin film made ofdiamond-like carbon or a lubricating thin layer made of a materialcontaining fluorine.
 21. The disk-substrate molding die of claim 20,wherein the lubricating thin layer has a surface roughness of 0.2micrometer or less as a center roughness.
 22. A disk-substratefabricating method, wherein a disk substrate is fabricated using thedisk-substrate molding die of claim
 1. 23. The disk-substratefabricating method of claim 22, wherein the fabrication of the disksubstrate is performed by injection molding.
 24. The disk-substratefabricating method of claim 22, wherein the disk substrate is made ofthermoplastic resin.
 25. The disk-substrate molding die of claim 15,wherein the surface of the second low heat-conductivity member or thecoating layer is covered with a thin film made of diamond-like carbon ora lubricating thin layer made of a material containing fluorine.